Diabetes mellitus is the third leading cause of death in the U.S. and the leading cause of blindness, renal failure, and amputation. Diabetes is also a major cause of premature heart attacks and stroke and accounts for 15% of U.S. health care costs. Approximately 5% of Americans, and as many as 20% of those over the age of 65, have diabetes.
Diabetes results from the failure of the β-cells in the islets of Langerhans in the endocrine pancreas to produce adequate insulin to meet metabolic needs. Diabetes is categorized into two clinical forms: Type 1 diabetes (or insulin-dependent diabetes) and Type 2 diabetes (or non-insulin-dependent diabetes). Type 1 diabetes is caused by the loss of the insulin-producing β-cells. Type 2 diabetes is a more strongly genetic disease than Type 1 (Zonana & Rimoin, 1976 N. Engl. J. Med. 295:603), usually has its onset later in life, and accounts for approximately 90% of diabetes in the U.S. Affected individuals usually have both a decrease in the capacity of the pancreas to produce insulin and a defect in the ability to utilize the insulin (insulin resistance). Obesity causes insulin resistance, and approximately 80% of individuals with Type 2 diabetes are clinically obese (greater than 20% above ideal body weight). Unfortunately, about one-half of the people in the U.S. affected by Type 2 diabetes are unaware that they have the disease. Clinical symptoms associated with Type 2 diabetes may not become obvious until late in the disease, and the early signs are often misdiagnosed, causing a delay in treatment and increased complications. While the role of genetics in the etiology of type 2 diabetes is clear, the precise genes involved are largely unknown.
Insulin is made exclusively by the β-cells in the islets of Langerhans in the pancreas. During development, the islet cells, including the β-cells, develop from an undifferentiated precursor within the growing pancreatic bud. As the bud grows, the undifferentiated cells form into ducts, and it is these cells that function as precursors. Duct cells appear to retain the capacity to differentiate into islet cells throughout life, and in some circumstances when the pancreas is damaged, new islet cells can form from the duct cells. Unfortunately, islet cell regeneration does not appear to occur when the islet cells alone are damaged, such as in type 1 diabetes.
This developmental process is clinically relevant for several reasons. First, the formation of islet cells and especially β-cells is necessary in order to make insulin and control energy metabolism. If the process of β-cell development is in anyway impaired, it predisposes that individual to the later development of diabetes. Therefore genes involved in this process are candidate genes for neonatal diabetes, maturity onset diabetes of the young (MODY) or type 2 diabetes. The sequence of these genes could be used to identify individuals at risk for the development of diabetes, or to develop new pharmacological agents to prevent and treat diabetes.
Second, as discussed above, insulin production is impaired in individuals with diabetes. In type 1 diabetes the impairment is caused by the destruction of the β-cells, while in type 2 diabetes, insulin production is intact, but inadequate. Treatment of type 1 diabetes, as well as many cases of type 2 diabetes, may involve replacement of the β-cells. While replacement of β-cells may be accomplished in several ways, the development of new β-cells from precursor cells, either in culture or in vivo in the patient, would be the most physiologic. To do this, the molecules that control β-cell differentiation are needed.
For these reasons, the diabetes field has spent considerable effort in attempts to identify islet precursor cells, and to develop methods for differentiating beta-cells in vitro. To date this has been largely unsuccessful. The present invention addresses this problem.